Without Start:

Ctrl+numbers (1 to 7) - switch to screen. Same as Start+numbers,
   but more convenient for XEGS and 1200XL users.
UNTESTED: 1200XL F1-F4: without Shift, switch to screens 1-4. With
    shift, F1-F3 = screens 5-7 and F4 is same as Start+A.
Help (XL/XE only) - same as Start+A.

Hold down Start, *or* press and release Escape, and:

1-7 - switch screens
Up Arrow (without Control) - scroll current screen up. Pressing
  any key while scrolled up will scroll back down.
Tab - switch to last displayed screen
Left/Right Arrows (without Control) - previous/next screen.
A - switch to active screen. [TODO: if no active screen, move
    left one screen... maybe? would this be helpful or annoying?]
E - show full editbox (instead of screen). You can edit in this
    mode, but pressing Enter, switching screens, or anything else
    that normally hides the edit box will switch back to normal
    screen + status bar display.
S - show status (hide edit box)
L - list all screens (number and name).
N - in channel screen, do a /names #channel. in [server], last
    channel with traffic. no effect elsewhere.
P - in a query screen, do a /ping <nick>. in [private], last
    nick who PMed. no effect elsewhere.
Q - create screen for last PM (in [private]) or last channel
    message (in [server]). Does nothing in screens 2-7.
T - /topic #channel
W - in query, /whois the user. in PM, last user who PMed.
    in a channel or [server], no effect.
X - close screen (but do not part channel).
Ctrl-X - in a channel screen, /part the channel and close the
    screen. no effect elsewhere.
Ctrl-D - disconnect from server (without sending a /quit). This
    *doesn't do anything useful* for users, it only exists as a
    debugging aid (so I can test the automatic server ping code).
    This will likely be removed before the first beta.

If you press Escape by accident, pressing it again gets you out of
"start key toggle" mode.

Future plans:

J - join last channel you were invited to.
